The present invention relates to a friction reduction technology based on a microstructure of sliding surfaces of mechanical components relatively slidable. More specifically, this invention relates to a surface roughness microstructure of sliding elements of a sliding structure for a reciprocating internal combustion engine, which is adapted to reduce friction loss in the reciprocating internal combustion engine.
The sliding elements of reciprocating internal combustion engines includes a crankshaft, a bearing metal, a piston skirt, a cylinder wall and the like. In a related art, a sliding surface of the crankshaft has a microstructure in which deep valleys are continuously formed over a wide region of its base surface having fine projections and recesses. In another related art, a sliding surface of the piston skirt has ten-point mean roughness Rz of not less than 20 xcexcm which is prescribed in JIS B 0601-1994. Owing to the surface roughness, even when the sliding surface of the piston skirt suffers from wear, microscopic valleys formed on the sliding surface still remain and act as oil retention recesses. The effect of oil retention can improve antiseizure properties of the piston skirt under insufficient lubricating condition thereof and high-revolution and high-load condition of the internal combustion engine. Further, in a related art, a sliding surface of the cylinder wall has a cross-hatched microstructure in which deep valleys are continuously arranged over a wide area to form intersecting grooves.
U.S. Pat. No. 6,095,690 discloses sliding bearing elements for use in reciprocating internal combustion engines, which have lubricating oil pockets on the sliding surfaces. The lubricating oil pockets have predetermined depths and are provided in predetermined regions. This related art aims to improve antiseizure properties of the sliding bearing elements under insufficient lubricating condition of the sliding bearing elements and high-revolution and high-load condition of the internal combustion engine.
In general, the thickness of a lubricating oil film formed between the relatively moving sliding surfaces which have such a surface roughness as described above and are subjected to load, is determined depending on balance between an amount of the lubricating oil introduced into a clearance between the sliding surfaces upon relative motion of the sliding surfaces and an amount of the lubricating oil leaked or discharged from the clearance due to pressure caused by the load. Upon being discharged from the clearance, the lubricating oil flows along the sliding surfaces which define the clearance, and a large amount of the lubricating oil flows out from a portion of the clearance in which the flow resistance is relatively small. Here, the flow of lubricating oil is a laminar flow. The amount of the laminar flow varies in proportion to cube of the clearance allowing the laminar flow to pass therethrough. Therefore, if such a portion of the clearance continuously extends in a direction of the flow of lubricating oil over a wide region of the sliding surfaces, the flow resistance at the portion of the clearance will remarkably decrease.
In addition, if the crankshaft described in the related art is used with the bearing metal having microscopic oil retention recesses on the sliding surface, the continuous microscopic deep valleys of the sliding surface of the crankshaft will be fluidly connected with the microscopic oil retention recesses of the sliding surface of the bearing metal. Similarly, in the combination of the cylinder wall as described in the related art and the piston skirt having microscopic oil retention recesses on the sliding surface, there is a tendency that fluid connection occurs between the continuous microscopic deep valleys of the sliding surface of the cylinder wall and the microscopic oil retention recesses of the sliding surface of the piston skirt. When the lubricating oil is discharged from the clearance between the sliding surfaces of these sliding elements, the lubricating oil selectively flows through the region of the sliding surfaces where the deep valleys and the oil retention recesses are fluidly connected. This causes reduction of the flow resistance so that the thickness of the lubricating oil film decreases. As a result, a shear force of the lubricating oil becomes larger to thereby increase friction loss in the internal combustion engine.
There is a demand to eliminate the above-described problems in the related arts. An object of the present invention is to provide a sliding structure for a reciprocating internal combustion engine which is capable of suppressing reduction of flow resistance to the lubricating oil flowing between opposed sliding surfaces of two sliding elements to thereby maintain an appropriate thickness of the lubricating oil film between the sliding surfaces, reduce friction loss due to undesirable decrease of the lubricating oil film thickness, and then improve performance of the reciprocating internal combustion engine.
According to one aspect of the present invention, there is provided a sliding structure for a reciprocating internal combustion engine, comprising:
sliding elements relatively slidable via lubricating oil therebetween, the sliding elements including sliding surfaces opposed to each other and lubricated with a laminar flow of the lubricating oil, the sliding surfaces defining a first clearance therebetween from which the laminar flow of the lubricating oil leaks,
wherein, assuming that the sliding surfaces are completely smooth surfaces defining a second clearance therebetween from which the laminar flow of the lubricating oil leaks, a first surface integral T of an inverse number of the first clearance is smaller than a second surface integral Tsmooth of an inverse number of the second clearance under condition that flow resistances to the laminar flows of the lubricating oil which leak from the first and second clearances are equal to each other with respect to pressure gradient in a direction of leakage of the laminar flows of the lubricating oil.
According to a further aspect of the present invention, there is provided a sliding structure for a reciprocating internal combustion engine, comprising:
sliding elements relatively slidable via lubricating oil therebetween, the sliding elements including sliding surfaces opposed to each other and lubricated with a laminar flow of the lubricating oil, each of the sliding surfaces including a base surface with microscopic irregularities having a maximum height t, and a plurality of dimples formed in a separate relation to each other on the base surface,
wherein, if the sliding surfaces are completely smooth surfaces defining a minimum clearance therebetween an amount of the lubricating oil introduced into the minimum clearance upon relative motion of the sliding elements balances with an amount of the lubricating oil discharged therefrom by pressure generated between the sliding elements, the minimum clearance is larger than a sum of the maximum heights of the microscopic irregularities of the base surfaces and smaller than an average value of maximum depths of the dimples of at least one of the sliding surfaces, and an average value of maximum diameters of the dimples of one of the sliding surfaces is smaller than an average value of minimum distances between the dimples of the other of the sliding surfaces.
According to a still further aspect of the present invention, there is provided a sliding structure for a reciprocating internal combustion engine, comprising:
a cylinder wall including a first sliding surface;
a piston skirt reciprocally moveable relative to the cylinder wall, the piston skirt including a second sliding surface opposed to the first sliding surface; and
a piston ring mounted to the piston skirt, the piston ring including a third sliding surface opposed to the first sliding surface;
each of the first, second and third sliding surfaces including a base surface with microscopic irregularities having a maximum height t, and a plurality of dimples separated from each other by the base surface,
wherein an average value d of maximum diameters of the dimples of the first and third sliding surfaces is not more than an axial length of the piston ring,
wherein a relationship between the average value d of maximum diameters of the dimples of the first sliding surface, an average value LS of minimum distances between the dimples of the second sliding surface, and an average value LR of minimum distances between the dimples of the third sliding surface is expressed as follows:
d less than LS and d less than LR, and 
wherein the maximum height t of the microscopic irregularities of the base surface is smaller than a smaller one of minimum clearances generated between the first and second sliding surfaces and between the first and third sliding surfaces, the minimum clearances being determined assuming that the first, second and third sliding surfaces are completely smooth surfaces, and that an amount of the lubricating oil introduced into the minimum clearances upon relative motion of the cylinder wall, the piston skirt and the piston ring balances with an amount of the lubricating oil discharged from the minimum clearances by pressure generated between the cylinder wall, the piston skirt and the piston ring under a maximum operating time condition of the reciprocating internal combustion engine.
According to a still further aspect of the present invention, there is provided a sliding structure for a reciprocating internal combustion engine for automobiles, comprising:
a cylinder wall including a first sliding surface defining a cylinder bore, the first sliding surface including a base surface with microscopic irregularities having a maximum height of not more than 1 xcexcm, and a plurality of dimples separated from each other by the base surface, the dimples having a depth ranging from 1 xcexcm to 50 xcexcm and an average value of maximum diameters thereof which is not more than 50 xcexcm;
a piston skirt reciprocally moveable within the cylinder bore, the piston skirt including a second sliding surface opposed to the first sliding surface, the second sliding surface including a base surface with microscopic irregularities having a maximum height of not more than 5 xcexcm, and a plurality of dimples separated from each other by the base surface, the dimples having a depth ranging from 1 xcexcm to 50 xcexcm, an average value of maximum diameters thereof which is not more than 1 mm, and an average value of minimum distances therebetween which is larger than the average value of maximum diameters of the dimples of the cylinder wall; and
a piston ring mounted to the piston skirt, the piston ring including a third sliding surface opposed to the first sliding surface, the third sliding surface including a base surface with microscopic irregularities having a maximum height of not more than 1 xcexcm, and a plurality of dimples separated from each other by the base surface, the dimples having a depth ranging from 1 xcexcm to 50 xcexcm, the average value of maximum diameters thereof which is not more than 50 xcexcm, and an average value of minimum distances therebetween which is larger than the average value of maximum diameters of the dimples of the cylinder wall.
According to a still further aspect of the present invention, there is provided a sliding structure for a reciprocating internal combustion engine, comprising:
sliding elements relatively slidable via lubricating oil therebetween, the sliding elements including sliding surfaces opposed to each other and lubricated with a laminar flow of the lubricating oil, the sliding surfaces including base surfaces with microscopic irregularities having maximum heights t, respectively, and a plurality of grooves extending at an angle of not less than 45 degrees relative to a direction of the sliding motion of the sliding elements without intersecting, the grooves having an average value of maximum depths which is larger than the maximum heights t, the grooves having maximum widths of not more than a contact length CL in the direction of the sliding motion over which the sliding surfaces are in contact with each other via a film of the lubricating oil disposed therebetween,
wherein an average value b of the maximum widths of the grooves of one of the sliding surfaces is smaller than an average value of minimum distances between the grooves of the other of the sliding surfaces.
According to a still further aspect of the present invention, there is provided a reciprocating internal combustion engine, comprising:
a sliding structure including sliding elements relatively slidable via lubricating oil therebetween, the sliding elements including sliding-surfaces opposed to each other and lubricated with a laminar flow of the lubricating oil, the sliding surfaces defining a first clearance therebetween from which the laminar flow of the lubricating oil leaks,
wherein, assuming that the sliding surfaces are completely smooth surfaces defining a second clearance therebetween from which the laminar flow of the lubricating oil leaks, a first surface integral T of an inverse number of the first clearance is smaller than a second surface integral Tsmooth of an inverse number of the second clearance under condition that flow resistances to the laminar flows of the lubricating oil which leak from the first and second clearances are equal to each other with respect to pressure gradient in a direction of leakage of the laminar flows of the lubricating oil.